I have been to Son Bonnet on a number of occasions and always found it a delightful experience. It cost me €38 for a 3 day stay the last time, compared to the main airport which cost €180 for the same length of stay.
Customs are possible by arrangement, but I generally stop at Perpignan on the way so am coming from Schengen. Make sure you have all of your and the aircraft documents in order as the Civil Guarda are hot on inspection. I have all mine in a binder with an index and the index has Spanish translations so he can go direct to the page he wants. This strategy means a five minute inspection unlike the guy who came in behind me in a swiss aircraft who chose to argue and was still being inspected when we had fuelled and got a taxi!!
Security is fine, it's like fort knox and you need a pass to get airside but no problem.
They will bring you in across the VOR on the North of the Island and fly you low level along the entire length of the Island on a long final parallel to the main airport arrivals. Very scenic trip!
All in all not a bade airport and far less hassle than the main airport.